Ah yes, this is a topic I am a lot of thoughts for. Anime is truly a love/hate relationship I have. Some I adore, some I cannot stand.
Overrated:
Death Note: The most overrated series I've seen. I gave up on it about 20 episodes in because of how incredibly stupid the writing was, especially with the main characters. Two perfect boys that do no wrong (L is "weird" but he still does everything else right), massive conveniences to make the characters meet and their constant one-uping of each other became tiresome so quick, because it is just one Gary-Stu outwitting another Gary-Stu again and again. This show is the epitome of Japanese cliches in this genre in my book. I will never understand it's popularity.
Jojo's Bizarre Adventures Stardust Crusaders: Speaking of good ol cliches, look no further than this. My first and currently only experience with the Jojo franchise, it makes me not want to see anything more. While season one has some good moments, overall the series is a mountain of immaturity and low brow humor that took me by complete surprise. Every character is of course some archetype that does the same thing over and over. Jotaro is always emo and edgy, Joseph is always stupid and saying random things in English (because that's...funny I guess?) and of course the French guy wants all the sex. Creativity was left on the drawing board I suppose. Then it peaks with insanely stupid villains that are often made to look like idiots and most make no sense. The main baddy, Dio, apparently hired a baby to fight the main characters. Amazing. There's even an absurd fakeout on a main character's death that made zero sense and destroyed what should have been a powerful. The writers set their own work on fire with their own writing. The humor is the worst offender overall. A show that is stamped with an MA rating for some pretty brutal stuff also has fart jokes and an entire episode dedicated to shallow sex jokes. Honestly, the more I've thought about it, the more I almost want to put this as my most number 1 hated in anime. I think some of the fights hold it up but not by much.
Pokemon: Simple. I love most of the games, but the anime, especially the original, is awwwwwwwful. I loved it when I was young, but holy balls is it stupid. It truly is a sight to behold nowadays just how bad it all is. I will always appreciate the designs it introduced to the franchise and it set that standard, actually watching it is painful though. The more the games try to be like the show, the worse they get, because the show is bad.
DBZ: Another where I love the games, but not so much the show. I don't hate it or anything, but it isn't some masterpiece like it's current status would imply.
Underrated? Honestly, I am not sure. I am not aware of a whole lot that I've seen being extremely overlooked. Except for maybe Deadman Wonderland but that too has a following. I say it is underrated because the anime only got one season. Whyyyyyyyy. I was very much into the first season, the finale of it brought out emotion I didn't expect. The music was stellar as well. I wish so much that we could see more of it in anime form.